The process of separating the components of a gas mixture by pressure swing adsorption (PSA) comprises, in general, a series of steps including the basic steps of adsorption and bed regeneration. The PSA process is generally carried out in an elongated vessel having an inlet and an outlet at opposite ends of the vessel, and containing a layer of adsorbent which adsorbs one or more of the components of the gas mixture more strongly than it adsorbs one or more other components of the mixture. During the adsorption step the gas mixture is introduced into the vessel through the vessel inlet and is passed through the vessel at elevated pressure, whereupon a fraction of the gas mixture enriched in the more strongly adsorbed component(s) is adsorbed by the adsorbent, while another fraction, enriched in the less strongly adsorbed component(s), passes through the vessel and is discharged from the vessel through the outlet as nonadsorbed product gas. As the adsorption step proceeds, adsorption fronts delineating the forward end of each component of the adsorbed fraction form and advance toward the outlet end of the vessel. The adsorption step is terminated before the most advanced adsorption front reaches the outlet end to prevent adulteration of the nonadsorbed product gas with more strongly adsorbed component(s). Subsequently, the adsorbent in the vessel is regenerated by permitting the vessel to depressurize by releasing gas from the vessel, generally through its inlet. The desorbed gas fraction may be recovered or disposed of, as desired. The adsorption and desorption steps are desirably repeatedly performed in cyclical fashion so that the process is substantially continuous.
A number of improvements have been made to enhance the performance of the above-described two-step process, which, by itself, is highly inefficient and results in the production of low volume, low purity product. Thus, the process is generally carried out commercially in a battery of adsorption vessels arranged in parallel and operated out of phase with one another to approximate a continuous process. In a particularly useful embodiment of a multiple bed system, a pair of adsorption vessels are arranged in parallel and operated 180 degrees out of phase with one another, such that one vessel is in adsorption service while the adsorbent in another is undergoing regeneration. The pressure in the vessel during the adsorption step (adsorption pressure) is usually atmospheric or above, for example, in the range of about 1 to about 20 bar, absolute, while that in the vessel undergoing bed regeneration is reduced to a value somewhat below the adsorption pressure.
An improvement which markedly improves the efficiency of a two bed system is the transfer of void space gas, i.e. the gas contained in the interstices between the particles of adsorbent, from the vessel which has just completed the adsorption step to the vessel which has just completed the bed regeneration step. This step is usually referred to herein as "equalization", since the pressure in the bed being depressurized and that in the vessel being repressurized tend to approach a common value. In practice however, the equalization step is not carried out to actual pressure equalization, since this could cause desorption of the adsorbed component in the bed being depressurized and transfer of this component to the vessel undergoing repressurization.
Bed equalization between a pair of parallel-operated adsorption beds can be carried out in a number of ways. In one procedure, fluid communication between the outlets of the two beds is established, and void space gas flows from the outlet end of one vessel to the outlet end of the other (outlet-to-outlet equalization). In another procedure, the inlets of the two beds are connected, permitting void space gas to flow from the inlet end of one vessel to the inlet end of the other vessel (inlet-to-inlet equalization). In a third procedure, the outlet of the vessel undergoing depressurization is connected to the inlet of the vessel undergoing repressurization (outlet-to-inlet equalization). Combinations of these procedures are also practiced. A highly efficient bed equalization procedure is simultaneous outlet-to-outlet and inlet-to-inlet equalization. This procedure has the advantages of minimal disturbance of the beds, rapid gas transfer and ensuring that the highest purity void space gas in the first bed is transferred to the outlet end of the second bed, which minimizes adulteration of the nonadsorbed product gas from the second bed when it goes into adsorption service. These procedures are discussed in more detail in U.S. Pat. No. 4,925,461.
U.S. Pat. No. 5,346,536 discloses a PSA process for separating nitrogen from oxygen that is carried out in two or more parallel-arranged adsorption vessels and whose cycle includes a step in which a portion of the gas being transferred from one vessel to another during bed equalization is vented from the system.
It is very difficult or impossible to attain perfect separation of the components of a gas mixture by PSA. However, because even small enhancements of product yield and/or product purity can have a considerable impact on the overall efficiency of a PSA process, improvements which will enhance the yield and purity of the product gases from PSA processes are constantly sought. The present invention provides a novel cycle which enhances the purity of the nonadsorbed gas stream produced by a PSA system.